The listings all have 'Payment Methods Accepted' section and you can only pay by those means. If personal check was not listed under that section as you say in your message here, then you cannot pay by check.
Payment Methods Accepted is a standard section in all listings and it must have appeared on their ad. Any payment type not listed is absolutely not a viable option of payment, as you put it.
When they said they preferred money order/cashier check they meant it is the best choice for them out of all the options they accept (they may also have accepted cash for example, but still preferred money order or cashier check. It doesn't mean they have a preference but will accept any type of payment.)